The present invention is directed toward the introduction of starting molten flux to the bottom of a crucible in an electroslag melting furnace and more particularly to the introduction of starting molten flux from the top of the crucible.
The electroslag melting process was first invented, developed and put into full production by R. K. Hopkins in the United States during the period between 1930 and 1960. This process employs a consumable electrode which is immersed in a pool of molten slag supported at the top of the resultant solidifying ingot enclosed within a cold-walled mold or crucible.
Alternating (or sometimes direct) current flows down the consumable electrode through the slag, down the ingot and back to the power supply. Preferably, the current flows back to the power supply in a coaxial manner to the top of the crucible such as shown in co-pending application Ser. No. 616,365, filed Sept. 24, 1975 now U.S. Pat. No. 4,032,705. This current, normally in the range of 1,000 amps per inch of ingot diameter, drops from fifteen to forty volts across the slag (or flux) pool thereby producing hundreds of kilowatts of melting power which consumes the tip of the electrode.
As a result of the foregoing, molten metal droplets form on the immersed electrode tip, detach themselves and fall through the molten flux pool to the ingot which is forming there below. As the metal droplets pass through the flux pool, they undergo chemical refinement. Progressive solidification of the ingot formed by this method leads to the physical isotropy and high yield associated with all consumable electrode processes.
As is known in the art, most electroslag ingots of 24 inch diameter and larger are started by pre-melting a slag of suitable chemistry and pouring a six to eight inch deep pool of this molten slag into the bottom of the crucible. The electrode tip is then immersed to a depth of half an inch or so into this molten pool. The melting current flowing through the molten flux raises its temperature until the electrode begins to melt.
Molten flux (or slag) starting, as this technique is known, gives much higher utilization of the consumable (electroslag) furnace and better ingot yield than "dry" or cold starting because ingot bottom losses are minimized.
In the past, many methods have been attempted or proposed for the effective introduction of molten starting slag to the bottom of the crucible. One such system includes a means for introducing this molten flux through a conduit formed by the intersection of the crucible stool and the bottom flange of the crucible. Such a system is shown, for example, in U.S. Pat. Nos. 3,679,089 and 3,736,124.
As is demonstrated by these prior patents, it has heretofore been considered impractical to introduce molten slag through the very narrow annular space formed between the outside diameter of the electrode and the inside diameter of the crucible (a space generally restricted to lateral width of 11/2 to 3 inches. This was true because the molten slag had a tendency to solidify on the outside surface of the electrode and the inside surface of the crucible thereby impairng melting, sharply affecting the accuracy of electrode weighing systems and tending to produce ingots of poor quality or abort the start.